Audible and audible-visible alarms are widely used, especially in commercial, office and institutional buildings, as elements of fire safety installations. Both audible and audible-visible alarms have a sound generator, such as a horn, bell or siren, and associated electronic circuitry for driving the sound generator. Audible-visible alarms have, in addition to the sound generator and its driving circuitry, a light source, which is universally a strobe lamp, and electronic circuitry for firing the lamp. It is well known, per se, to provide the sound generator and driving circuitry in the form of a circuit board module, which has input terminal connectors for connecting the module to signal wires that originate at a remote control unit. The control unit receives input signals from heat and smoke detectors and sets off alarms in selected zones that may become hazardous to building occupants as a result of a fire indicated by inputs from particular detectors. Similarly, strobe lamp firing circuits and the strobe light are commonly produced as circuit board electronic modules and strobe light units, the latter consisting of a mounting board and a light reflector and a strobe lamp or lamps mounted directly to the board.
Audible and audible-visible alarms are almost always mounted on walls using electrical backboxes. Many previously known alarms have had mounting plates that serve as both mountings for the alarm modules and covers that conceal the modules. Because there are several sizes and configurations of backboxes in common use, the mounting plates have been produced in different sizes and configurations for use with the different styles of backbones. Recently, the owner of the present invention has introduced alarms with universal mounting plates that have multiple sets of screw holes, each set being used with a different backbox style. The multiple screw holes make it desirable to have a cover, separate from the mounting plate on which the alarm module is mounted, to conceal the screw holes. The covers for the universal mounting plates snap on to the mounting plate and thus not only conceal the multiple screw holes and the screws but have no visible screws, which improves the appearance of the alarm as compared to mounting plates that also serve as covers and in which the screws are visible. The universal mounting plates and covers referred to above are described and shown in U.S patent application Ser. No. 08/524,865, filed Sep. 7, 1995, which application is incorporated into the present specification for all purposes.
The strobe light units of audible-visible alarms must protrude from the front of a mounting plate or cover so that the light can propagate not only away from the wall but in directions parallel to the wall. Accordingly, the covers for the universal mounting plates have a hole, through which the strobe light unit can protrude from the mounting plate. Audible alarms have no protruding element, so a cover without a hole is provided. The need for different covers for audible and audible-visible alarms increases costs in the form of design and tooling expenses and inventory stocking and control. The possibility of mistakes in ordering and delivery can produce delays in installation. If an installer does not match the units and the covers at the job site, he or she will have to exchange the non-matching covers before the job can be completed. Meanwhile, the distributor who supplied the installer will probably have a mismatch in the stocks of units and covers. In a large job, the installer will have to allocate matching units and covers for each alarm site. Mismatches will result in lost time when the installer has to go to a storage location and correct a mismatch.